The long-term goal of this project is to obtain a better understanding of the molecular mechanisms involved in cellular immune phenomena. Our present efforts are primarily concentrated on the cytotoxicity of human natural killer (NK) lymphocytes for certain cultured tumor cell lines. We have developed a distribution-free kinetic method for the analysis of natural cytotoxicity in vitro that allows for the precise quantification of the frequency and lytic activity on NK cells in peripheral blood samples. Using this approach, we have demonstrated that the enhancement of natural cytotoxicity by interferon is due to an increase in the frequency of competent NK cells and not an increase in their lytic activity. In addition, we have analyzed the specificity of natural cytotoxicity by this kinetic approach. Results of this study support the existence of multiple NK target antigens that are partially shared by susceptible NK target cells. Recently, we have also developed an enzyme-release assay for natural cytotoxicity that appears to have several advantages over the standard Chromium-release assay, including reductions in assay time and cost without a loss in sensitivity or accuracy. These techniques are now being applied to delineate the biochemical mechanism of NK activation by interferon and other agents, as well as further efforts to isolate and characterize NK target antigens and the structures on NK cells that recognize these antigens.